TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractFiber optic distributed temperature monitoring of downhole conditions is an emerging technology that can be used to obtain important reservoir temperature information where conventional methods have failed. Due to the development of fiber coatings and improved deployment techniques, fibers are now used reliably up to 480 °F in Aera Energy's California field. Case 1is an installation on tubing in rod pump wells has been useful in detecting early steam breakthrough and is expected to reduce the need for temperature observation (TO) wells in the future. Case 2 is a permanent installation on the outside of casing in TO wells which is used to determine accurate reservoir temperature profiles. In one example, the fiber has shown fluid migration behind pipe which was remediated successfully before the steam reached the surface. Case 3 is a failed attempt to use fiber to continuously monitor multilateral horizontal well temperatures to determine relative fluid contributions of the wellbores without shutting in the well.